The present application relates to a liquid crystal display device which can display in both an isotropic phase temperature range and a nematic phase temperature range, and a driving method of the same. More particularly, the present application relates to a liquid crystal display device having a wide operable temperature range, in which in an isotropic phase temperature range of a liquid crystal forming material, a display with high speed response can be performed using an induction and disappearance phenomenon of a nematic phase at the time of electric field application and non-application, and in a nematic phase temperature range, a display can be performed by using a change of orientation state of directors at the time of electric field application and non-application.
Hitherto, since a liquid crystal display device has features of light weight, low profile and low power consumption as compared with a CRT (Cathode Ray Tube), it is often used for display in an electronic equipment. When related art liquid crystal display devices are classified according to a method of applying an electric field to a liquid crystal layer, a vertical electric field system one and a lateral electric field system one are known. The liquid crystal display device of the vertical electric field system applies a substantially vertical electric field to liquid crystal molecules by a pair of electrodes disposed on both sides of a liquid crystal layer. As the liquid crystal display device of the vertical electric field system, the device of a TN (Twisted Nematic) mode, a STN (Super Twisted Nematic) mode, a VA (Vertical Alignment) mode, a MVA (Multi-domain Vertical Alignment) mode, an ECB (Electrically Controlled Birefringence) mode or the like is known.
In the liquid crystal display device of the lateral electric field system, a pair of electrodes insulated from each other are provided on the inner surface side of one of a pair of substrates disposed on both sides of a liquid crystal layer, and a substantially lateral electric field is applied to liquid crystal molecules. As the liquid crystal display device of the lateral electric field system, the device of an IPS (In-Plane Switching) mode in which the pair of electrodes are not overlapped with each other when viewed in a plane, and the device of an FFS (Fringe Field Switching) mode in which they are overlapped with each other are known.
In these related art liquid crystal display devices, the alignment direction of liquid crystal directors aligned in a specified direction is changed by the electric field, and the amount of light transmission is changed to display an image. The operation principle of the related art liquid crystal display device as stated above will be described with reference to FIGS. 7A to 7D.
FIG. 7A is a schematic sectional view of a related art liquid crystal display device of a vertical electric field system, which is a liquid crystal display device using, as an optical element, a change of optical phase difference occurring when an external electric field (voltage) is applied to a liquid crystal layer. FIG. 7B is a view showing a light transmission state in the liquid crystal display device. FIGS. 7C and 7D show orientation states of directors in the liquid crystal layer of the nematic liquid crystal layer having a positive dielectric constant anisotropy, and shows a voltage non-application state (FIG. 7C) and a voltage application state (FIG. 7D). Most related art liquid crystal display devices are used as the display devices by changing the orientation of directors of a liquid crystal, such as a nematic liquid crystal, at a temperature lower than transition temperature of nematic phase-isotropic phase.
As shown in FIG. 7A, in the related art liquid crystal display device, a liquid crystal layer is sandwiched between an array substrate AR and a color filter substrate CF, and a transparent electrode is formed on the liquid crystal layer side of each of the array substrate AR and the color filter substrate CF. A polarizing plate is disposed on the outer surface (opposite side to the liquid crystal layer) of each of the array substrate AR and the color filter CF, and a backlight light source is disposed on the outer surface of the polarizing plate on the array substrate AR side. As shown in FIG. 7B, light incident on the polarizing plate on the array substrate AR side from the backlight light source is converted into linearly polarized light, and a phase difference is given to the linearly polarized light when passing through the liquid crystal layer. Further, only the light parallel to the transmission axis of the polarizing plate on the color filter layer side passes through and is visually recognized.
The directors in the liquid crystal layer are aligned in, for example, a horizontal direction by the action of the alignment film formed on the surface of the transparent electrode in an electric field non-application state (see FIG. 7C), and is aligned in a vertical direction in an electric field application state (see FIG. 7D). As stated above, since the alignment state of the directors of the liquid crystal layer is changed between in the electric field non-application state and in the electric field application state, the phase of the light passing through the liquid crystal layer is changed. Thus, in the related art liquid crystal display device, the amount of light transmission is controlled by an interaction between the electric field formed by the pair of electrodes and the transmission axis of the polarizing plate, so that a specified image can be displayed.
Incidentally, in the liquid crystal display device of the lateral electric field system, although the pair of electrodes are formed on the array substrate AR, the device is not different from the liquid crystal display device of the vertical electric field system in that the amount of light transmission is controlled by an interaction between the electric field formed by the pair of electrodes and the transmission axis of the polarizing plate so that a specified image is displayed.
On the other hand, various compounds are known as the liquid crystal forming material. For example, 4-cyano-4′ pentylbiphenyl) (hereinafter referred to as “5CB”) expressed by the following chemical formula has a positive dielectric anisotropy, is solid at 24° C. or less, is liquid at 35° C. or higher, and exists in a liquid crystal state at a temperature between 24° C. to 35° C. That is, the 5CB performs a phase transition between a liquid crystal phase and a solid phase (isotropic phase temperature range) at about 35° C.

In the liquid crystal phase, the 5CB exists in the nematic phase. When the nematic phase is heated, a phase transition to the isotropic phase occurs discontinuously at about 35° C., and during that, there occurs a state (hereinafter referred to as a pseudo-isotropic phase) which is an isotropic phase optically and macroscopically, and microscopically indicates a nematic phase property.
Although the temperature range in which the pseudo-isotropic phase appears is about 1K and is very narrow, Non-patent document 1 listed below discloses that excellent electro-optical effects occurs as follows:
(1) when a polymer network is stretched in the nematic phase in which a chiral agent is mixed, the pseudo-isotropic phase in the case of no electric field macroscopically becomes the isotropic phase in a wide temperature range by the polymer network with random structure,
(2) when an electric field is applied, since dielectric anisotropy occurs in the pseudo-isotropic phase by electro-optical Kerr effect, optical anisotropy occurs, and when the electric field is removed, the state is quickly returned to the original state, and
(3) the response time at the time of electric field application-removal is an order of 10 μsec, and is very high when consideration is given to a fact that the response speed when the alignment direction of the related art nematic phase is changed is several msec or more.
Besides, Non-patent document 1 discloses that when the polymer network is stretched in a blue phase appearing in a narrow region between the chiral nematic phase and the isotropic phase, excellent electro-optical effects are obtained as follows:
(4) the inducing temperature range of the blue phase widens to 100 K or more,
(5) when an electric field is applied to the blue phase, a birefringent phenomenon appears by the electro-optical Kerr effect, and when the electric field is removed, the birefringent phenomenon disappears, and
(6) with respect to the response speed at the time of electric field application-removal, both a rising time and a fall time are 10 to 100 μsec, and is very higher than the response speed when the alignment direction of the related art nematic phase is changed.
Examples of the related art includes JP-A-11-183937 (Patent document 1), JP-A-20001-265298 (Patent document 2), JP-A-2007-323046 (Patent document 3) and Liquid Crystal, vol. 9, No. 2 (2006), pp. 83 to 95 (Non-Patent document 1).